Most cells get their energy from cellular respiration, a process that breaks down glucose molecules and harvests the chemical energy they contain. This illustration ...
Miller and Levine, Biology
From Biology, by Kenneth R. Miller and Joseph Levine, ©2002 by Pearson Education, Inc. Reproduced by permission of the publisher.
The Krebs cycle is the first of two stages of a process called cellular respiration, in which glucose is transformed into a usable form of chemical energy called adenosine triphosphate, or ATP. Although cellular respiration is a relatively complicated process, involving dozens of steps, its chemical equation is quite simple. It begins with the raw materials glucose and oxygen and yields carbon dioxide and water (both waste products) and free energy, some of which is captured and stored in usable form as ATP. The chemical equation for this conversion is C6H12O6 + 6 O2 ---> 6 CO2 + 6 H2O + energy (ATP).
This process cannot begin without glucose, a simple sugar molecule made up of 6 atoms each of carbon and oxygen and 12 atoms of hydrogen. However, all of us consume many types of food besides glucose. In fact, glucose is relatively uncommon in our diets. Over the course of a typical day, most people consume a combination of carbohydrates, fats, and proteins. So where does the glucose required to power cellular respiration come from?
The food we eat must be broken down before it ever enters the cell's mitochondria, where cellular respiration takes place. Breaking down complex carbohydrates into glucose is a relatively simple process. Complex carbohydrate molecules are made up primarily of multiple glucose molecules linked together. Enzymes in the stomach and intestines separate individual glucose molecules from one another early in the digestive process. In contrast, fats and amino acids, the molecules that make up proteins, have chemical structures that only vaguely resemble glucose. They contain carbon, hydrogen, and oxygen atoms, just like glucose, but in dramatically different ratios.
The liver is responsible for converting these molecules into glucose. It is also the place where excess carbohydrates are converted into a readily available but storable carbohydrate molecule called glycogen and into fat. The direction of these conversions depends on the level of glucose in the blood. When the concentration of glucose in the blood is low, the liver converts glycogen and fat (and in their absence, protein) into glucose. When blood glucose levels are high, the liver reverses the process, storing carbohydrates and maintaining fat and protein stores.
Cells obtain glucose from the blood, through the walls of capillaries nearby. These capillaries carry not only glucose but also oxygen and many other important nutrients. Once inside the cell glucose is absorbed by organelles called mitochondria. These important structures play host to the two stages of cellular respiration: the Krebs cycle and the electron transport chain. Combined, these chemical conversions and the raw material glucose that feeds them, produce the energy that drives nearly every cellular process in your body.